The Geometry of Ultra-Fast Outflows Probed by Soft X-ray Variability in PDS 456
Abstract
Constraining the location and geometry of ultra-fast outflows (UFOs) is essential for identifying where they are launched and how they are accelerated. We investigate the soft X-ray variability of the luminous quasar PDS 456 using simultaneous March 2024 observations with XRISM/Xtend and NuSTAR. A model-independent comparison between the flare and quiescent phases reveals spectral variability around 1 keV in the rest frame, while the hard X-ray spectral shape remains nearly unchanged. Broadband spectral fitting shows that the soft X-ray structure is well described by a partial-covering low-ionization UFO with log (ξ/(erg cm s−1)) ≃ 3.1 and vout ≃ 0.30c. Time-sliced spectral analysis further reveals significant variability in the covering fraction of this absorber. Interpreting this variability as transverse motion across the X-ray source, we constrain the crossing velocity to be vcross ≲ 5 × 10−3c and derive a lower limit on the absorber distance of r ≳ 4 × 103 Rg. This location is substantially farther out than the high-ionization UFO previously inferred at ~200-600 Rg, while the two phases have comparable outflow velocities. The resulting velocity-distance structure disfavors a self-similar magnetocentrifugal wind and instead suggests either radiation-pressure acceleration following a Castor-Abbott-Klein-like velocity law or compact magnetic acceleration through magnetic reconnection. These results demonstrate that soft X-ray partial-covering variability can provide a geometrical probe of UFOs and directly connect spectral variability to wind acceleration.
Versions
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BibTeX
@misc{sato2026geometry,
title = {{The Geometry of Ultra-Fast Outflows Probed by Soft X-ray Variability in PDS 456}},
author = {Riki Sato and Kouichi Hagino and Toshiya Iwata and Ehud Behar and Valentina Braito and Chris Done and Misaki Mizumoto and James N. Reeves and Rozenn Boissay-Malaquin and Pierpaolo Condó and Luigi C. Gallo and Adam G. Gonzalez and Alfredo Luminari and Aiko Miyamoto and Ryuki Mizukawa and Hirokazu Odaka and Atsushi Tanimoto and Makoto Tashiro and Francesco Tombesi and Yerong Xu and Satoshi Yamada and Tahir Yaqoob and Aya Bamba},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.28034},
eprint = {2608.28034},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.28034},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The Geometry of Ultra-Fast Outflows Probed by Soft X-ray Variability in PDS 456
AU - Riki Sato
AU - Kouichi Hagino
AU - Toshiya Iwata
AU - Ehud Behar
AU - Valentina Braito
AU - Chris Done
AU - Misaki Mizumoto
AU - James N. Reeves
AU - Rozenn Boissay-Malaquin
AU - Pierpaolo Condó
AU - Luigi C. Gallo
AU - Adam G. Gonzalez
AU - Alfredo Luminari
AU - Aiko Miyamoto
AU - Ryuki Mizukawa
AU - Hirokazu Odaka
AU - Atsushi Tanimoto
AU - Makoto Tashiro
AU - Francesco Tombesi
AU - Yerong Xu
AU - Satoshi Yamada
AU - Tahir Yaqoob
AU - Aya Bamba
PY - 2026
DA - 2026/08/28
PB - arXiv
DO - 10.48550/arxiv.2608.28034
UR - https://arxiv.org/abs/2608.28034
AB - Constraining the location and geometry of ultra-fast outflows (UFOs) is essential for identifying where they are launched and how they are accelerated. We investigate the soft X-ray variability of the luminous quasar PDS 456 using simultaneous March 2024 observations with XRISM/Xtend and NuSTAR. A model-independent comparison between the flare and quiescent phases reveals spectral variability around 1 keV in the rest frame, while the hard X-ray spectral shape remains nearly unchanged. Broadband spectral fitting shows that the soft X-ray structure is well described by a partial-covering low-ionization UFO with log (ξ/(erg cm s^−1)) ≃ 3.1 and v_out ≃ 0.30c. Time-sliced spectral analysis further reveals significant variability in the covering fraction of this absorber. Interpreting this variability as transverse motion across the X-ray source, we constrain the crossing velocity to be v_cross ≲ 5 × 10^−3c and derive a lower limit on the absorber distance of r ≳ 4 × 10^3 R_g. This location is substantially farther out than the high-ionization UFO previously inferred at ~200-600 R_g, while the two phases have comparable outflow velocities. The resulting velocity-distance structure disfavors a self-similar magnetocentrifugal wind and instead suggests either radiation-pressure acceleration following a Castor-Abbott-Klein-like velocity law or compact magnetic acceleration through magnetic reconnection. These results demonstrate that soft X-ray partial-covering variability can provide a geometrical probe of UFOs and directly connect spectral variability to wind acceleration.
ER -